For centuries, signs have been used to convey information such as commands, warnings and directions. Typically, earlier signs were letterboards posting the appropriate message. Once electricity became commercially viable, electric lighting was used to illuminate signs so that an observer could read the message at nighttime. Other signs were later developed which included electric lighting disposed within a light-transmissive sign housing to illuminate the sign both during daytime and nighttime hours for enhanced viewing of the message. Generally, these earlier signs conveyed one fixed message. With the advent of computers, signs have become more advanced in that variable messages could now be displayed on a single sign. Although there are many applications for variable message signs, one specific application is for highway use in order to convey messages to motorists. Usually, these messages warn motorists of forthcoming road conditions. Since road conditions can vary within brief periods of time, messages displayed on variable message signs must be changed accordingly so that motorists can have the latest and most accurate information of the road conditions lying ahead.
To effectively convey messages to motorists, the size of the message itself and positioning of the variable message sign are important considerations. In order to give a motorist adequate time to see and read the message, the letters on the sign must be sufficiently large. Typically, the alphanumeric characters of the message are eighteen (18) inches tall to provide sufficient legibility at a distance of at least eight hundred (800) feet from the variable message sign. Usually, the variable message sign is positioned either on a shoulder of the roadway or mounted onto a truss expanding above and across the roadway.
Coupled with a computerized controller, a display causes a desired message to appear on the variable message sign. The display comprises an array of signal elements that respond to commands from the computerized controller. Each of the signal elements in the array is typically either an electromechanical element or a light-emitting element. For example, one electromechanical element includes shutters and flip disks which alternate between a darkened state and a brilliant state to form the commanded characters in the message. The light emitting elements include ordinary light bulbs and light emitting diodes (commonly referred to as "LED's") which alternate between a light-on state and a light-off state to form the commanded characters in the message.
Particularly for roadway use, the display is usually enclosed in a sign housing to protect the signal elements and other working components of the variable message sign from adverse environmental conditions such as blowing dust and rain. In contrast, warm, sunny days can be detrimental to the operability of the variable message sign because radiant energy from inferred, ultraviolet and visible wavelength of sunlight is absorbed by the display and sign housing thus, generating heat. Heat can be detrimental to the signal elements and the other working components of the variable message sign. Absorption of radiant energy by the display and sign housing generates heat within the interior of the variable message display. Furthermore, operating electromechanical and light-emitting signal elements also generates heat within the interior. Temperatures within the interior during warm, sunny days can reach
160.degree. Fahrenheit and above. Such extreme temperatures causes degradation of the operability of the signal elements resulting in expensive repairs or pre-mature replacement.
Repairs and replacement of components used with variable message signs can be problematic. For example, one type of a variable message sign employs clusters of diodes encapsulated in a plastic visor tube which typically shields the diodes from direct ultraviolet sunlight but not scattered ultraviolet sunlight. The visor tube snaps into and out of an array board extending across the face of the variable message sign. If, for any reason, the cluster of diodes within the visor tubes becomes inoperable, a repairman might be required to be suspended from above the variable message sign to gain access to the inoperable cluster of diodes. Although exchanging the visor tube is simple, gaining access to the inoperable one can be difficult and dangerous in that the repairman could be suspended over a busy highway. Additionally, ultraviolet sunlight causes degradation of the plastic visor tube which could break under sever weather conditions. If broken, the visor tube may not be able to shield the diodes from direct ultraviolet sunlight which, in turn, accelerates degradation of the diodes.
A need therefore exists to provide improved variable message signs, for example, which can shield radiant energy produced by sunlight to reduce heat generation into the interior thereof. A need also exists to provide variable message signs which are better ventilated to exhaust heat from the interior of the sign housing. Another need is to provide variable message signs which are easier and safer to repair and maintain. The present invention addresses these needs and these advantages.